Automatic pilot



March 18, 1952 R. J. KUTZLER ET AL AUTOMATIC PILOT Filed June 27, 1945 Patented Mar. 18, 1952 AUTOMATIC PILOT Robert J. Kutzler and Theodore J. Wilson, Minneapolis, Minn.,

assignors to Minneapolis- Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application June 27, 1945, Serial No. 601,816

25 Claims.

Our invention relates generally to control systems and more particularly to motor operated control systems for aircraft wherein a crew member is provided with a manually operable means which may be used by him to control the operation of the motors and thereby control the aircraft in any one of several different manners.

It is customary for aircraft to be provided with manually operable means, such as the well known stick, which operates the control surfaces directly by means of cables. In addition, when a control system of the motor operated type is used to control the position of the control surfaces, it is often desirable to have a manually operable controller which may be used to control the operation of the motors; and it is this latter type of controller which is meant when we refer to a "manual controller, unless the other is clearly intended.

In a previous application Serial No. 570,712, filed December 30, 1944 now patent 2,471,821 dated May 31, 1949, by the present inventors, a manually operable control device for this purpose was shown and described, and a control system was shown which is suitable for use with that device. We have found that certain additional features are desirable in such a control system, and these features, relating more to the control of the control system, are shown and described in this present application. The manual controller used with this system may be either the type shown in our previous application or may be of the type shown in the application Serial No. 601,813 of Arthur S. Osborne now patent 2,481,776 dated September 13, 1949, filed the same date as this.

As shown in our previous application mentioned above, it is often desirable to have a manual control means available for use in conjunction with an automatic pilot so that the main control of the aircraft is provided by the automatic pilot, while changes in the path maintained by the automatic pilot may be provided by the manual control means. However, it is often desirable to be able to control the control surfaces of the aircraft directly and not indirectly, as through the autopilot,.in some cases where very precise control must be exercised as, for example, where an aircraft is in close proximity to other aircraft. In such cases it is very convenient to be able to use the servo motors of the autopilot system to operate the control surfaces, thereby relieving the pilot of the physical effort necessary to do this; and at the same time itis desirable for the pilot to be able to control the surfaces directly without the compensating or opposing effect of the gyroscope usually found in such autopilots.

In other cases it is desirable for the pilot to be able to control the elevation of the aircraft very accurately while at the same time permitting the autopilot to control the course of the plane, as for example in the case of aerial photographic mapping where it is quite important that the altitude remain constant despite updrafts or downdrafts in the atmosphere, while the autopilot ies the predetermined course.

Since the servo motors, manual control means, and other devices necessary to produce these results are present in the control system disclosed in our previous application, it will be very convenient if means may be provided whereby the various elements may be connected to form a control system having any of the desired characteristics.

It is therefore a major object of our invention to provide a control system which may be used in conjunction with an autopilot in an aircraft to provide operation of the control surfaces of the aircraft in accordance with the demands of the autopilot system as modified by the use of the manual control means, or to provide operation of the control surfaces in accordance with the movement of the manual control member without the counteracting effect of the gyroscopically or similarly stabilized members of the autopilot.

It is another object of our invention to provide a control system for an aircraft which has a manually operated control means normally eiective to cause the previously mentioned servo motors to position the control surfaces of the aircraft and change its angular position about either or both of two axes, but which may be instantaneously changed to render control about one of these axes ineffective.

It is a further object of our invention to provide a control system of the class described having a manual control means normally effective to cause the angular position of said aircraft about either or both of two axes to be varied, the control system being capable of rendering the operation of said manual controller ineffective to change the angular position of said aircraft about one axis, and materially reducing the effectiveness of the controller in the other plane.

It is still another object of our invention to provide a control system having four different types of control, the types being that in which the autopilot may be used alone with the manual control means wholly ineffective to modify the operation of the autopilot; that in which the autopilot may be used with the manual control means effective to modify the operation thereof to change the angular position of the aircraft about three axes; that in which the autopilot may be used to control the position of the aircraft about three axes while the manual ccntrol means is ineffective to modify the operation of the autopilot about two of these axes andv is effective, to a reduced degree, to modify the operation of the autopilot about the third axis; and that in which the operation of the autopilot as such is renderedl ineffective and the manual control means is effective to control the angular position of the aircraft about three axes.

These and other objects of our invention will become apparent from the following description of a preferred form thereof and from the drawing illustrating that form in which the single figure is a schematic wiring diagram of a control system incorporating our invention.

Referring now to the drawing, it will be seen that there are three control networks I8, II, and I2, designated aileron, rudder, and elevator networks, respectively. The networks I0, II, and I2 have outputterminals connected to amplifiers I3, i4, and I5, respectively, and these in turn are connected to servo motors I5, I1, and I8, respectively, whichinturn are connected by cables to the ailerons, rudder, and elevator (not shown) of the aircraft. It is to be understood that we do not claim the ampliers I3, I4, and I or the servo motors I6, I1, and I8 as our invention, since such amplifiers andr motors are well known in the art and may be of a type such as that shown in the patent to Whitman No. 17,942,587 or to Anschutz-Kaempfe No. 1,586,233. As shown in the drawing, the network I0 includes a bridge 2| and a voltagen dividing network22 connected in series therewith; and the output'cf these two is connected to the amplifier I --3 to operate the servo niotor I5.

-Bridga 2I includes a rebalancing potenti- 'meter23'having a wiper 24'which bears against a resistor 25, and a control potentiometer havingV awiper 21 which bears against a resistor 28. The resistors 25 and 28 are connected in parallel, and a source of power, such as a transformer having a primary winding 3I and a secondary winding 32, is connected to the resistors so that the customary Wheatstone bridge resultsfwith the wipers 24 and 21 forming the output terminals' thereof.

For the purposes which will become apparent later, it is often desirable to change the point on the resistor 25 of the potentiometer 23 at which the wiper 24 must be located in order to balance the bridge. For this purpose, .we provide a pair of centering potentiometers 33 and 34 connected in series with the rebalancing potentiometer 23 so that electrical energy owing fromthe secthe other resistor is simultaneously decreased.r

As a result, the total resistance in the series circuit measured from one end of the secondary I 32 of the transformer 30 through the potentiometers 33, 23, and 34 remains a constant as the wipers and 38 are moved, while the point at which wiper 24 must be located to balance the bridge for a given position of wiper 21 will vary v as the wipers 35 and 38 are moved. rlhe reondary winding 32 of the transformer 3U flows,

-for example, from the left-hand side of the winding 32, through the centering potentiometer 33, through the rebalancing potentiometer 23, through the centering potentiometer 34, and back to the right-hand side of the secondary winding. Potentiometer 33 includes a wiper 35 which bears against a resistor 31, and potentiometer 34 has a wiper 38 which bears against a resistor 39. Wiper 35is electrically connectedr to one .end of the resistor 31 and wiper 38; is simi-j larly connected to one end of resistor 39; and as balancing potentiometer 23 is operated by the servo motor I5, the wiper 24 of the potentiometer being positioned by the output shaft of the servo motor; and the potentiometer 25 is operated by a means whichv is responsive to the angular position of the aircraft in a plane passing athwartships of the craft. The responsive means 45 will usually take the form of a gyroscope having its axis vertical, but any other suitable means may be used if this seems desirable.

From wiper 21 of potentiometer 26, connection is made to the voltage dividing network 22 by means of conductor 45. The network 22 includes a potentiometer 46 having a wiper 41 which bears against a resistor 48, and the latter is in turn connected to a source of power such as a secondary winding 49 of the transformer 33 having a primary winding 3I. The conductor 45 is connected to the center tap of the resistor 48; and the wiper 41, which is operated by an azimuth responsive instrument 5I) such as a directional gyroscope, is connected to a voltage dividing potentiometer 55. The potentiometer 55 includes a wiper 56 which bears against a resistor 5l, and one end of this resistor is connected to the wiper 41, while the other end of the resistor is connected to the conductor 45.

It will thus be seen that whenever the wiper 41 is displaced from the center tap of the resistor 48, a voltage will appear between the wiper and the center tap, and this voltage will be applied across the resistor 51. Since one end of resistor 51 is connected to conductor 45, when wiper 55 is at that end of resistor 51, none of the voltage between the opposite end of resistor 51 and conductor 45 will appear between wiper 55 and conductor 45. Similarly, when wiper 56 is at the opposite end of resistor 51, all of the voltage appearing between the ends of resistor 51 will appear between wiper 55 and conductor 45. In this Way, potentiometer 55 provides a means for controlling the amount of the voltage developed between wiper 41 and conductor 45 which appears between wiper 56 and conductor 45, and therefore controls the effectiveness of the azimuth responsive instrument 53. From wiper 55, connection is made by conductor 55 to a fixe-:l terminal 5I of a servo relay 59 having an operating coil 52 which is normally in its deenergized condition. When the relay is deenergized, a movable contact 63 of the relay bears against the out contact 6I, and the latter is connected through the movable contact and a conductor 64 to one of the input terminals of amplifier I3. the other terminal of which is grounded at E5.

From the terminals of the secondary 32 of the transformer 30 in bridge 2l, a pair of conductors 58 and 69 lead to a trimmer potentiometer .15, having a wiper 1I which bears against a resistor 12. The conductors 08 and 09 are connected to opposite ends of resistor 12, thus connecting it in parallel with resistor 28 of potentiometer 2B; and the wiper 1| is connected to an in contact 13 of servo relay 59 by conductor 14.

It is thus seen that when the operating coil 62 of the servo relay 59 is deenergized, the amplier I3 is connected through the movable contact 63 and the out contact 9| to the network 22, and hence it is connected to the gyroscopically operated potentiometers and 20. When the operating coil 02 is energized, the amplifier I3 is connected through the movable contact 03 and the in contact 13 to the bridge 2| in such a manner that the gyroscopically operated potentiometers 40 and 2-5 will have no effect upon any signal which the amplifier i3 is receiving.

Shown below the network |0 is an adjusting network 00 which includes a pair of potentiometers 8| and 82 each having a wiper 83 and 84, respectively, which bear against resistors 85 and 80, respectively. The 'resistors 85 and 89 are connected in parallel by conductors 81 and 88, and conductor 88 is grounded at 09. Wiper 24 of potentiometer 23 in bridge 2| is connected by conductor 90 to wiper 83 of the potentiometer 8 I, and conductor 01 of the adjusting network 80 is connected by conductor 9| to the switch blade 92 of a switch 93.

Switch 93 is one of a plurality switches 94, 95, 96, and 91 which are ganged or otherwise connected together for simultaneous operation. These switches are selectively adjustable to any one of four positions, and these positions are numbered at switch 94. It will be seen that the switches are in position 2, and it will be seen that in this position, switch blade 92 of switch 93 bears against a contact 93 which is grounded at 99. In position 4, switch blade 92 bears against Contact In which is also grounded at .i effects would be the same if network |22 instead 99; and in position I, switch blade 92 bears against contact |0| which is not otherwise connected.

In position 3, switch blade 92 bears against contact |02 which is connected by conductor |03 to a movable contact |04 of a transfer relay |09.

The transfer relay |09 has an operatingl coil |05, and when the coil is in its normal or deenergized condition, the movable contact |04 bears against an out contact |06 from which connection is made by conductor |01 to a turn control network |08.

The turn control network |08 includes a potentiometer I|0 having a grounded wiper I|| which bears against a center tapped resistor I I2. f

The resistor I I2 is connected to a suitable source of power such as the secondary winding ||3 of a transformer ||4 having a primary winding ||5. The conductor |01 is connected to the center tap of the resistor I2, and thus as wiper I I0 is moved away from the center tap, a voltage will appear between Conductor |01 and ground. The potentiometer ||0 may conveniently be one of the po- Atentiometers included in a control device ||1,

against an in contact |20 which is connected by conductor I 2| to a network I 22 identical in all respects with network |08. The network |22 includes a potentiometer |23 having a grounded wiper |24 which bears against a center tapped resistor 25. The latter is connected to a secondary winding |26 of the transformer ||4, and conductor |2| is connected to the center tap of resistor |25. Like potentiometer H0, potentiometer |23 may be one of the potentiometers in a control device |21, such as that of Osborne previously mentioned; and it will be apparent that when operating coil |05 is energized, movement of wiper |24 away from the center tap of resistor |25 will cause a voltage to appear between movable contact |04 and ground; while under the same conditions the movement of wiper of potentiometer I0 will have no effect upon the voltage appearing between the movable Contact |04 and ground. When the operating coil 05 is deenergized and movable Contact |04 bears against the out contact |06, movement of wiper II away from the center tap of resistor ||2 will cause a voltage to appear between movable contact |04 and ground; and the movement of wiper |24 of potentiometer |23 will be ineffective to change this voltage. If the pilot of the aircraft is provided with a control device ||1 and the copilot of the aircraft is provided with a similar device |21, the operating coil |05 may be utilized to transfer control from the pilot to the copilot and vice versa, the pilot having control when the coil is deenergized and the copilot having control when the coil is energized. Since the operation of network |08 is identical to that of network 22, further description and explanation will be made considering the network |08 connected to conductor |03 through out contact |06 and movable contact |04, though the operation and all of network |08 were connected to the conductor |03.

As previously mentioned, when switch blade 92 of switch 93 is in position I, no connection is made from contact |0| and conductor 9| is connected to movable contact |04 of the transfer relay |09 through resistor I6. Consequently, any voltage appearing between conductor |01 and ground is transmitted to adjusting network through resistor ||0, the magnitude of the voltage applied to the network 80 being reduced thereby. When switch blade 92 is in position 2, it bears against contact 98 which is grounded at 99, and conductor 9| is thereby grounded so that any voltage appearing between conductor |01 and ground is impressed across resistor IIB. Conductor 9|, and hence conductor 81, is at ground potential, as is conductor 88, and as a result no effect of the voltage between conductor |01 and ground is noted at network 80. In position No. 3, switch blade 92 bears against contact |02 and thereby connects conductor 9| through conductor |03 to movable contact |04, thereby shorting resistor I|6. As a result, the full voltage developed by network |08 is applied to network 80 and there is no diminution of this voltage by reason of its passing through a resistor. When switch blade 92 is in position 4, it bears against contact |00 which is also grounded at 99, similarly to contact 98; and the effect of switch 93 in positions 2 and 4 is therefore the same.

From the foregoing, it may be seen that with switch blade 92 in position 3 any voltage appearing between conductor |01 and ground will be conducted to conductor 81 and there impressed across the resistors 65 and 86 of potentiometers Bl and 82, respectively. By adjusting the position of the wipers 83 and 8d, any desired portion of this voltage may be selected; and from wiper 83 the voltage is transmitted by conductor 90 to bridge 2|.

As previously mentioned, switch 94 is operated simultaneously with switch 93, the switch blade |30 of switch 94 isconnected to a source of power, andthe No. contact is connected to the operating coil 62 of the servo relay 59 first described by conductor |3|. The other terminal of the operating coil 62 is grounded at |32 to complete the circuit to the source of power; and thus when the switch 94 is in position the operating coil of `the servo relay 59 is energized and all the movable contacts, such as contact 63, are moved by it to their in positions.

`A voltage circuit for the input to amplier I3 may now be traced. With the switches 93 and 94 in position i, and starting at grounded wiper I |i in the network SES, the circuit may be traced from the wiper Hi to conductor |91 and out contact |06, 'through movable contact iti and resistor H6 to conductors 6| and 81. Any voltage appearing between conductor 9i and ground will be impressed across resistors 35 and B6 in adjusting network 89, and a portion of this voltage will appear on wiper 36 and be transmitted by conductor 99 to wiper 2i! or" the rebalancing potentiometer 25 in the bridge 2|. From resistor 25,' the Ycircuit divides and one branch goes through potentiometer t3 and conductor 68 to one side ci potentiometer 19, while the other branch goes through the potentiometer 3i and conductor 69 to the other side oi potentiometer 10. From potentiometer 10, the circuit continues through the wiper il thereof to the in contact 'i3 of the servo relay 59, through the movable Contact 63 thereof and thence to one of the input terminals oi amplifier i3, the other input terminal of which is grounded at 65 to complete the circuit.

With switches 2S and 9d in position 2 as shown, the operating coil 62 of the servo relay 59 is deenergized and all the movable contacts controlled by it are in their out positions. Conductor 9|, which is connected to switch blade 92, is grounded through contact 98 at 99, and hence any voltage which appears between conductor 59'! and ground is impressed across resistor i6 and has no effect upon adjusting networked. Since conductor 9| is grounded, conductor 8i, to which it is connected, is likewise grounded, as is conductor 88; and wiper 83 of potentiometer 8| is therefore at ground potential, as are conductor 9c and wiper 2li of potentiometer'23 in network 2 From wiper 24, the circuit bek traced through resistor 25, potentiometers 3S and 34, potentiometer 26 and wiper 2l' thereof to conductor 45, and then through potentiometer i6 of voltage dividing network 22, to one side of potentiometer 55, the other side oi which is connected to conductor '125. From wiper 56 of potentiometer 55, connection is made through'conductor 56 to the out contact 9| of the servo relay, then through movable contact 63 thereof to conductor et through amplifier |3, returning`to ground at 65.

With switches 93 and 9S in position 3, operating coil 62 of servo relay i9 is -deenergized, permitting movable contact 63 tobear against iixed contact 6| and resistor H6 is shorted by switch 93; and hence starting at grounded wiper the circuit may be traced Vthrough conductor |91 to "ut contact |06 of the'transfer relay |09,

through movable contact |013 and conductor |03 to contact |62 and switch blade 92 of switch 93 and then through conductor 9| to conductor 81. From conductor 8? the circuit may be traced through potentiometer 55| and the wiper S3 thereof to conductor 99 and wiper 24 of potentiometer 23 of the bridge circuit 2|. From there it continues through potentiometers 33 and 36 to potentiometer 26 and the wiper 21 thereof to conductor i6 and through potentiometers 46 and 55. as previously described, to wiper 56, out contact 6|, movable contact 63, and conductor 64 to amplifier i6, from whence it returns to ground 65 and back to the grounded -wiper With switches 93 and 9d in position 4, switch blade 92 is grounded through contact |00 at 99; and hence conductor 6i is grounded as it was when the switches 93 and 9d were in position 2. The remainder oi the circuit is likewise identical with that prevailing when the switches were in position 2 and hence will not be repeated here.

lt will be seen that network i includes a bridge i5| and a voltage dividing network |52. ncluded in bridge |9i is a rebalancing potentiometer |53 having a wiper |56 which bears against a resistor |55 and which is positioned by motor One end of resistor L55 is connected through a potentioineter |56, which has a wiper |51 bearing against a resistor 58, to one terminal of a source of power such as a secondary winding |59 of a transformer having a primary winding 3|. The other end oi resistor |55 is connected through a potentiometer E56, similar to potentiometer |58, and having a wiper i6| bearing against a resistor 62, to the opposite end of the secondary winding |519. Wiper 51 of potentiometer |56 is connected to one end of the resistor |58 so that as the wiper is moved along the resistor, more or less oi the resistor is shorted. Similarly, wiper li is connected to one end of resistor |62 so that as this wiper is moved along its resistor, more or less or" the resistor is likewise shorted. The wipers E5? and i6| are mechanically connected together so that as one wiper is moved to short less of its resistor, the other wiper is moved to short more of its resistor; and thus as the wipers |51 and iti are moved, the resistance from one end of secondary winding I5| through potentiometer E56, potentiometer |53, and, potentiometer |59 to the other end of the secondary winding 59 remains a constant. To complete the bridge, a potentiometer |65, which has a wiper |66 bearing against a resistor |61, is .connected so that its resistor is connected to the terminals of the secondary winding |59. It will thus be seen that a Wheatstone bridge is provided, with wipers 55e and |66 forming the output terminals thereof. Wiper .|54 is operated by the servo motor ii', and wiper |66 .is operated by the azimuth responsive instrument 50.

From wiper 66, connection is made to network |52 by conductor |19. Included in network |52 is a potentiometer |1| having a wiper |12 which bears against a resistor |13, the ends of which are connected to a secondary winding |69 of a transformer 39 havinga primary winding 3|. Conductor H9 is connected to the center, tap of resistor |73, and the output of potentiometer Ill, as measured between the center tap and wiper |12, is applied to a voltage dividing potentiometer I'ifi having a wiper |15 which bears against a resistor |16, one end of which is connected to wiper |12 and the other end of which is connected to conductor |10. From wiperl |15, connection is made by conductor |18 to one of the input terminals of amplier |4, the other input terminal of which is grounded at |19. In addition to these connections, connection is made by conductor |80 from conductor |10 to another terminal of the servo relay 59, the conductor |80 being connected to an in contact |8| which is engaged by a movable contact |82 when the operating coil 62 is energized. From movable contact |82, connection is made to conductor |18 by conductor |83; and thus when the servo relay 59 is energized, the voltage divider bridge |52 is shorted by the contacts |8| and |82.

Wiper |54 of the rebalancing potentiometer |53 in the bridge |5| is connected by conductor |84 to wiper 84 of the potentiometer 82 in the adjusting network 80. Potentiometer 62 performs a function similar to that performed by potentiometer 8|, namely, the securing of the desired portion of the voltage appearing between conductor 81 and ground and transmitting that portion of the voltage to the bridge A voltage circuit for the input of the amplier |4 may now be traced. With the selector switches 93 and 94 in No. 1 position, starting at grounded wiper in the network |08, the cir cuit may be traced through conductor |01, out contact |06, movable contact |04 of the transfer relay 09, through resistor ||6 to conductor 9|, and then to conductor 81 as previously described. From conductor 81, the circuit continues through a portion of resistor 86 of the potentiometer 82, through the wiper 84 thereof and conductor |84, to wiper |54 of the potentiometer |53 in bridge |5|. From wiper |54 the circuit may be traced through resistor |55 of the potentiometer |53, through potentiometers |56 and |60 and resistor |61 of potentiometer |65, to the wiper |66 thereof. From wiper |66, connection is made through conductor |10, conductor |80 to in contact |8| of the servo relay 59 and movable contact |82 thereof (relay 62 being energized with switch 94 in No. 1 position), through conductors |93 and |18 to one of the input terminals of amplifier` I4, the other terminal of which is grounded at |19 to complete the circuit.

When switches 93 and 94 are in position No. 2, switch blade 92 is in engagement with contact 98 which is grounded at 99, and hence conductor 9|, connected to switch blade 92, is grounded, thereby placing both conductors 91 and 88 at ground potential. Wiper 84 of potentiometer 82 is therefore at grounded potential, as is conductor |84 and wiper |54 of the rebalancing potentiometer |53. From potentiometer |53 the circuit may be traced through potentiometers |56 and |50 and through resistor |61 of potentiometer |65 and through the wiper |66 thereof. From wiper |66, connection is made by conductor |10 through potentiometer |1| and wiper |12 thereof to one side of resistor |16 of potentiometer |14, the other side of which is connected to potentiometer |10. From wiper |15, connection is made by conductors |11 and |18 to the input of amplifier |4, the other input terminal of which is grounded at |19 to complete the circuit.

With switches 93 and 94 in position No. 3, the input circuit may be traced from the grounded wiper of the network |08 to conductor |01, through "out contact |06 and movable contact |04 of transfer relay |09 to conductor |03 and contact |02 and then to switch blade 92 and conductor 9| tov conductor 81. From conductor 81, the circuit continues through resistor 86 of potentiometer 82, and the wiper 84 thereof to conductor |84 and wiper 54 of rebalancing potentiometer |53. From potentiometer |53 the circuit continues through potentiometers |56 and |60 to the resistor |61 of potentiometer |65 and to the wiper |66 thereof. From wiper |66 connection is made by conductor |10 to one side of the resistor |16 of potentiometer |14, and also to potentiometer |1I, the wiper of which is connected to the opposite end of resistor |16. From wiper |15 of potentiometer |14, the circuit is continued through conductors |18 and |11 to one of the input terminals of amplifier |4, the other -input terminal of which is grounded at |19 to complete the circuit.

With switches 93 and 94 in position 4, the voltage circuit for the input of amplifier |4 is the same as that traced when the switchesare in position 2, and hence it will not now be repeated. n"

Elevator network |2 includes a bridge 20| 'and a voltage dividing network 202. Included in bridge 20| is a rebalancing potentiometer 203 having a wiper 204 which bears against a resistor 205. One end of resistor 205 is connected through a potentiometer 206, having a wiper 201 bearing vagainst a resistor 208, to one terminal of a suitable source of power such as a secondary winding 209 of a transformer 30 having a primary winding 3 The other end of resistor 205 is connected through a potentiometer 2|0, having a wiper 2|| which bears against a resistor 2|2, to the other terminal of the secondary winding 209. Wiper 201 is connected to one end of resistor 208 and wiper 2|| is connected to one end vof resistor 2|2, so that as these wipers are moved across their respective resistors, more or less of the resistor will be shorted. Wipers 201 and 2|0 are mechanically connected together in such a manner that as more of one resistor is shorted, less of the other resistor is shorted; and thus the total resistance from one terminal of the secondary winding through potentiometers 206, 203, and 2|!) to the other terminal of the secondary winding remains a constant as the wipers 201 and 2 0 are varied.

To complete the bridge circuit, a potentiometer 2|5, having a Wiper 2| 6 which bears against a resistor 2|1, is connected so that the ends of the resistor are connected to the terminals of the secondary winding 209. A Wheatstone bridge, similar to bridges 2| and |5|, is thus provided with wipers 204 and 2|6 forming the output terminals thereof. Wiper 204 is connected to and positioned by servo motor 8, and wiper 2| 6 is positioned by the vertical gyroscope 40 in a manner hereinafter described.

Network 202 includes a center tapped potentiometer 220 having a wiper 22| which bears against a resistor 222, the opposite ends of which are connected together by a conductor 223. The ends of the resistor 222 are connected by conductor 224 to one terminal of a suitable source of power such as a secondary winding 225 of a transformer 30 having a primary winding 3|. The center tap of the resistor 222 is connected by a conductor r 226 to the other terminal of the secondary winding 225. An adjusting potentiometer 230, which has a wiper 23| bearing against a resistor 232, has one end of the resistor connected by conductor 233 to wiper 22| of potentiometer 220, and the other end of the resistor 232 is connected by conductor 234 to conductor 226. Wiper 22| is operated by the vertical gyroscope 40 in a manner hereinafter described, and conductor 226 is convil nected to wiper 2|6 of the potentiometer 2|5 in the bridge circuit. Wiper 23| of potentiometer 230 is connected by conductor 235 to an out contact 236 of the servo relay 59; and movable contact 231 of that relay, which may make connection with contact 236, is connected by conductor 238 to one of the input terminals of amplier |5, the other terminal of which is grounded at 239.

-Connected in parallel with potentiometer`2|5 in the bridge 20| is a potentiometer 250 having a wiper 25| which bears against a resistor 252. One end of resistor 252 is connected by conductor 253 to one end of secondary winding 209, while the other end of resistor 252 is connected by conductor 254 to the opposite end of transformer 209. Wiper 25| is connected .by conductor 255 to in contact 256 of the servo relay 59 and hence may be contacted by movable contact 231 to connect wiper 25| toamplifier I5 through conductor 238.

To complete the elevator circuit, a network 260, comparable to the network |68, is provided which includes a potentiometer 26| having a grounded wiper 262 bearing against a center tapped resist-or 263. The ends of the resistor 26| are connected to a suitablesource of power such as the secondary winding 264 of transformer 4; and the center tap of the resistor is connected by conductor 265 to an out contact 266 of the transfer relay. When the operating coil 205 of the transfer relay |09 is deenergized, a movable contact 210 of that relay bears against the out contact 266. Movable contact 210 is connected to one end of a resistor 21|, the other end of which is connected by conductor 212 to Iwiper 204 of rebalancing potentiometer 203 in bridge 20 I.

As previously described, the potentiometer ||0 in the network |08 is preferably one of the potentiometers included in the control device I1 which may be of the type disclosed in the Osborne application previously mentioned. In this application, there is disclosed a second potentiometer which may be simultaneously and independently operated while operating the rst potentiometer ||0. This second potentiometer may be used as potentiometer26| in the network 260; and the dotted lines connecting controller I I1 with potentiorrieter` 26| and with potentiometer ||0 indicate that these potentiometers are controlled by the control device and are cap-able of simultaneous and independent operation. As previously men- 2 tioned, the pilot of the aircraft is provided with a control device ||1, and the copilot is provided with a similar control device |21. Since the control devices ||1 and |21 are identical, control device |21, in addition to having the potentiometer |23 previously described, also has a potentiometer 280 having a grounded wiper 28| which bears against a center tapped resistor 262. The ends of resistor 262 are connected to a source of power such as afsecondary winding 263 of transformer I4 to form a network 286; and the center tap of the resistor is connected by conductor 284 to in" contact 265 of the transfer relay |09 which may be contacted by the movable contact 210.Y As wiper 28| is displaced from the center position, a voltage will appear between conductor 284 and ground`23| and this voltage will bel transmitted to the in contact 285. Similarly, when wiper 262 is displaced from center position along resistor 263, a voltage will be developed between conductor 265Vand ground. However, only one of these voltages will be applied to resistor 21|l and this voltage will be determined by the position of the transfer relay |69. Since both network 260 and network 286 are identical l2 and since only one of the networks may be connected to resistor 21| at any given time, in the description which follows it will be assumed that network 260 is connected to resistor 21| through out contact 266 and movable contact 210.

To complete the circuit associated with the input of the elevator amplifier i5, a conductor 290 is connected from conductor 212 to a resistor 22|, the other end of the latter being connected to a fixed contact 262 located at position in switch 96. The switch blade 263 of switch 96 is grounded at 264; and thus when switch 66 is in position 4, conductor 212 and wiper 205| of potentiometer 203 are grounded through resistor 29|. Under these conditions, any voltage appearing between movable contact 210 of the transfer relay |09 and ground wiil be impressed across resistors 21| and 29 in series; and these resistors will act in the manner of the well known voltage divider to determine the voltage of wiper 204 with respect to ground.

A voltage circuitfor the elevator amplifier l5 may now be traced. Starting at grounded wiper 262 of network 260, and with switches 24 and 96 in position the circuit may be traced through potentiometer 26| to conductor 265, out contact 266 and movable contact 216 of the servo relay, resistor 21|, conductor 212, and -wiper 204 to resistor 205 of the rebalancing potentiometer 203 in bridge 20|. From one side of resistor 205, connection is made through potentiometer 205 and conductor 253 to one end of resistor 252 of potentiometer 256. From the other end of resistor 205, connection is made through potentiometer 2|0 and conductor 252 to the opposite end of resistor 252. From potentiometer 256, connection is made through wiper 25| thereof and conductor 255 to in contact 256 of the servo relay 59 and from there through movable contact 231 and conductor 238 to one of the input terminals of amplifier i5 and then through the other input terminal to ground 236.

With switches 96 and 66 in position No. 2, the circuit may be traced from grounded wiper 262, through potentiometer 265 andconductor 265 to contact 266 of the transfer relay |69 and then through movable contact 216 thereof, through resistor 21| and conductor 212 to wiper 204 of the rebalancing potentiometer 203. From potentiometer 203 the circuit continues through potentiometers 206 and 2|6 to potentiometer 2|5 of bridge 20|, from whence connection is made by wiper 2|6 to conductor 226. Conductor 226 is connected through conductor 234 to one end of resistor 232 of potentiometer 230, the other end of which resistor is connected through conductor 233 and potentiometer 226 to conductor 226. From lpotentiometer connection is made by wiper 23| thereof and conductor 236 to contact 236 of the servo relay 59; and from there the circuit continues through movable contact 231 and conductor 238 to one of the input terminals of amplifier I5, the other input terminal of which is grounded at 239 through which the circuit is completed back to grounded wiper 262.

With switches 04 and 66 in position 3, the voltage circuit for the input of the elevator amplifier l5 is identical with that just described when the switches are in position 2, and that description will not be repeated here.

With switches 64 and 56 in position 4, the circuit may be traced from grounded wiper 262 of network 260 through potentiometer 26| thereof and conductor 265 to out contact 266 and movable contact 210 of the transfer relay |09. AFrom movable contact 210, one circuit may be traced through resistor 21|, conductor 212, conductor 290, resistor 29|, contact 292, and switch blade 293 to ground 294 and thence back to grounded wiper 262. An additional circuit may be traced through resistor 21| and conductor 212 to wiper 204 of the rebalancing potentiometer 203 in the bridge 20|. From potentiometer 203 the circuit continues through potentiometers 206 and 2|0 to potentiometer 2|5, and then through wiper 2|6 thereof to conductors 226 and 234 to one connection of potentiometer 230, the other connection of which is connected through potentiometer 220 to wiper 2|6. From potentiometer 230, connection is made by wiper 23| and conductor 235 to out contact 236 of the servo relay 59, from which the circuit is completed through movable contact 231, conductor 238, the input circuit of amplifier 5, and ground 239 to grounded wiper 262.

It will be noted that in the description just preceding, the elevator circuit is similar in all respects with switches 94 and 96 in position 2 and in position 3. It will later be shown that it is undesirable for any voltage to be developed between wiper 262 and conductor 265 no matter how far the wiper is displaced from the center tap, when switches 94 and 96 are in position 2. To prevent such a voltage from appearing, the primary winding II of the transformer |I4 which provides the energization for all of the networks |08, |22, 260, and 280, is connected to a switch 91, one of the series of ganged switches, in such a manner that the primary winding ||5 is not energized when the switches are in position 2, but is energized when the switches are in position I, 3, or 4.

To do this, one of the terminals of the primary winding |I5 is connected by conductor 30| to one of the output terminals 302 of an inverter 303; and the other terminal of the primary winding is connected by conductor 304 to contacts 305, 306, and 301 of switch 91, corresponding to positions I, 3, and 4 of the ganged switches. Switch blade 308 of switch 91 is connected by conductor 309 to the other output terminal 3|0 of the inverter 303; and thus when switch 91 is in position I, 3, or 4, the transformer ||4 will be energized and a voltage will be impressed across the resistors in the bridges |06, |22, 260, and 280. 'Ihe transformer 30, shown with different secondaries as appearing in bridges and networks 2|, 22, |5I, |52, 20|, and 202, is connected directly to the output terminal of inverter 303 by conductors 3|| and 3|2 which, however, for the sake of clarity, are not shown as connecting to each representation of primary 3|.

Power for this control system is supplied from batteries-320 in the aircraft, or by a generator (not shown), one terminal of which is grounded at 32|, and the other terminal of which is connected to the master switch 322 of the aircraft. From switch 322, power is usually taken through a fuse 323 to the control system master switch 324; and from the latter, a conductor 325 carries power to one of the input terminals 326 of the inverter 303, the other input terminal 321 of which is grounded at 323. Conductor 330 connects conductor 325 to switch blade |30 of switch 94, thereby providing the power to energize the coil 62 of the servo relay as previously described; and a conductor 33|, connected either to conductor 330 or to conductor 325, furnishes power to amplifiers I3, I4, and l5, the current returning through the ground connections 65, |19, and 239 previously mentioned and shown in conjunction with the input circuits of these respective amplifiers.

To operate the transfer relay |09, the coil |05 is connected by conductor 332 to resistor 333, and the other terminal of this resistor is connected by conductor 334 to conductor 325. The opposite terminal of coil |05 is connected by conductor 335 to a push button switch 336 which is grounded. Conductor 335 is also connected to an in contact 331 of the relay, which is contacted by grounded movable contact 338 when the coil I 05 is energized. When switch 336 is momentarily closed, the circuit through coil |05 is completed and all the movable contacts associated with the relay are moved so as to bear against the in contacts. Under these conditions, the circuit through coil |05, which was originally completed through switch 336, is also completed through contacts 331 and 338; consequently, when the switch 336 is then released, the coil |05 remains energized through contacts 331 and 330. To deenergize the coil |05, conductor 345 is connected to conductor 332 and to a push button switch 346 which, when closed, grounds conductor 345. When this is done, coil |05 is shorted and the full voltage of the source 320 is applied between the ends of resistor 333. When coil |05 is shorted, the movable contacts operated by it move to their out position, and upon releasing the switch 346, the contacts remain in that position.

Since switch 336 will cause the transfer relay |09 to move the movable contacts of the relay to the position where the copilots control device |21 is effective to cause operation of the ampliners I3, I4, and 5, switch 336 is located so as to be readily accessible to the copilot. Similarly, since switch 346 will insure that the movable contacts of the relay are in their out positions, where the pilots control device I I1 is elective to control the operation of amplifiers I3, I4, and I5, switch 345 is located so as to be convenient for his use. In addition, because of this arrangement, the pilot is able to override the copilots control at any time he desires. For the maximum convenience, switches 346 and 336 may be mounted in the ends of the pilots and copilot's control devices ||1 and |21, respectively.

Considering now the position responsive instruments 40 and 50, the instrument 40 will generally consist of a so-called vertical gyroscope, i. e., a gyroscope whose spin axis is maintained perpendicular to the surface of the earth. Such gyroscopes are well known in the art, and we do not claim the gyroscope as such as our invention. Such a gyroscope, when mounted in an aircraft, will indicate when one wing is lower than the other, i.v e., the plane has changed its angular position about its roll axis; and it may also be used to indicate when the plane is diving or climbing, i. e., when the plane has changed its angular position about its pitch axis. These, of course, are two separate axes, and separate mechanical means may be operated by the movement of the gyroscope about these two axes. In the control system illustrated, wiper 2 I6 of potentiometer 2|5 in bridge 20| is connected by a mechanical linkage 350 to the position responsive device 40 in such a manner as to be moved when the plane changes its angular position about its pitch axis. Similarly, wiper 21 of potentiometer 26 in bridge 2 wiper |12 of ,potentiometer |1I in network |52, and wiper 22| of potentiometer 220 in bridge 202 are connected by a linkage 35| to the position responsive device 40 in such a manner that any change of the angular position of the aircraft about its roll axis will cause these wipers to be moved across their respective resistors.V

The position responsive device 53 is one which is effective to measure changes in azimuth and may take the form of a horizontal or directional gyroscope or a compass of any suitable type. With this device, any change in the angular position of the aircraft in azimuth will immediately be indicated by relative movement of two members therein. This movement is used to control a linkage 352 to move wiper lI.' of potentiometer 43 in the network 22 and wiper |55 of potentiometer |35 in bridge I5|. As will be described hereinafter, it is the purpose of this azimuth responsive device 59 to maintain the craft in a predetermined heading, so that there is no change in its angular position in azimuth. This is usually a very desirable characteristic, but when it is desired to turn the aircraft while the latter is under the control of the system herein described, it is necessary that some means be used to hold the mechanical linkage 352 and with it the wipers 41 and |66 in their centered position so that when the turn is completed, the aircraft will then remain on the new heading. Later, it will also be seen that it is desirable that the azimuth responsive device have no control over the aircraft when the switches 93 to 91 inclusive are in position No. I.

To hold the linkage 352 in centered position under any of these conditions, we have placed a so-called locking means 353 between the azimuth responsive device 59 and the linkage 352. This locking device forms no part of our present invention and may be merely a magnetically operated brake which engages the mechanical linkage 352 while a clutch permits the responsive member of the azimuth responsive device to slip with respect to the linkage 352. While this system works very satisfactorily, many other suitable systems are available.

To energize the lock 353 when the switches 93 to 91 inclusive are in position No. I, a movable contact 354 of the servo relay 59 is connected by conductor 355 to one of the input terminals of the locking device 353, the other terminal of which is grounded. When operating coil 62 of the servo relay is energized, movable contact 354 is moved into engagement with in contact 353 and is thereby connected to conductor I3! which is simultaneously energizing coil 32.

When wiper III of potentiometer III) in the network |38 is displaced from the center tap of resistor ||2, it will later be shown that the aircraft tends to turn. Under these conditions, it is desirable to have the linkage 352 locked in center position so that it will not be moved by the azimuth responsive instrument 50 as the aircraft turns. To operate the locking means 353, a switch 390 is incorporated in the control device ||1 so Vthat any time wiper III is displaced from center position, the contacts of the switch are closed. This is indicated in the drawing by the pointer-like member 36| which is mechanically connected to the wiper I and which bears against a spring leaf contact 362 to urge the latter away from the other contact 333 of the switch 330. Contact 353 is connected by conductor 334 to the switch blade 365 of switch 95; and when the latter is in position No. 3, it bears against acontact 336 which is connected by conductor 351 to conductor 325. Contact blade 362 is connected by conductor 319 to an out contact 31| of the transfer relay |39; and the movable contact 312 associated therewith is connected by conductor 313 to operate the locking mechanism 353.

A similar switch 313 is provided in the copilot's control device |21, and one of its contacts is connected by conductor 315 to contact 363 of the pilots switch 353. The other contact is connected by conductor 316 to in contact 311 of the transfer relay |35 and is connected to conductor 313 when the operating coil |35 of the transfer relay is energized.

It will thus be seen that when the gang switches 93 to 91, inclusive, are in position I, the locking means 353 is. energized by means of the servo relay 59, and the contacts 353 and 356 thereof. When the switches are in positions 2 and 5, the locking means is not energized by the servo relay 59 or by the swiches 339 or 314.. However, when the switches are in position 3, the locking means 353 will be energized by the switch 339 or 314, depending upon whether the coil |05 of the transfer relay |39 is energized or not, whenever the corresponding wiper is displaced from its center position.

To prevent the operation of the servo motors I6, I1, and I8 by amplifiers I3, Ill, and I5, respectively, when the operating conditions may not be satisfactory, switches 335, 33|, and 382 are placed in the circuits connecting amplifiers I3, I4, and I5 to servo motors I3, I1, and I8 respectively. When any switch is open, its amplifier has no effect on its servo motor, even though a signal may be impressed upon the input circuit of the amplier.

It is to be understood that we do. not claim the amplifiers I3, Ill, and I5 or the servo motors I5, I1, and I8 as our invention, nor do we claim the angular position responsive member 4l] nor the direction responsive instrument 53 as included within this invention, since such ampliers, servo motors, and responsive members are well known in the art. The patents to Whitman No. 1,942,587 and Anschutz-Kaempfe No. 1,586,233 are cited as examples of patents disclosing amplifiers and servo motors suitable for use in our control system. rlhe application of the present inventors Serial No. 570,712, filed December 3G, 1944, now Patent No. 2,471,821, shows various features of the control system described, including particularly the bridges and networks 2| and 22, 15| and |52, network 83, bridge 29| and network 252. Furthermore, the application of Arthur Osborne, previously mentioned, shows the design and construction of control devices that may be use as control devices II1 and |21 previously described. The subject matter of this application is not claimed as part of this invention except insofar as they combine and cooperate` with other portions of the circuit described to produce new and useful functions as particularly pointed out in the claims.

Operation Assume now that the craft is airborne, that switches 322 and 323 are closed, and that the ganged switches 53, 93, 95, 93, and 91 are in position 2. Switches 333, 33|, and 332 are in open position and the aircraft has been trimmed by the usual manual trim tabs so that it is ilying in the desired heading with its wings level and is neither gaining nor losing altitude. Under these conditions, the amplifiers I3, Ill, and I5 are energized, and bridges and networks 2|, 22, |5I, |52, 23|, and 252 are energized by their re- 17 spective transformers, and transformer |I4 is not energized. The mechanical linkage 352, the wiper 41 of potentiometer 4B in the network 22, and wiper |66 of potentiometer |65 in bridge |5|, controlled by the azimuth responsive instrument 50, are centered and held in that position. Network is then brought into a condition of balance by moving the wipers 35 and 38 of the potentiometers 33 and 34, thereby reducing the output of the amplifier I3 to zero.

In order for the wings to remain level, the ailerons must be in a definite position, and this position will be transmitted by the cables to the aileron servo motor I6 which will in turn position the wiper 24 of the potentiometer 23. Since, as previously mentioned in the description, conductor 81 of the adjusting network 80 is grounded when the ganged switches 93 to 31, inclusive, are at position 2, and since conductor 88 is grounded, wiper 24 will likewise be at ground potential. Since the plane is flying with its wings level, the vertical gyroscope 40 will operate the mechanical linkage 35| to center the wiper 21 of the potentiometer 2B; and since the wiper 41 of the potentiometer 46 in network 22 has been mechanically centered, all control wipers except wiper 24 are in centered position. If wiper 24 is not centered mechanically with respect to resistor 25, it may be electrically centered by adjusting wipers 36 and 38 of potentiometers 33 and 34 until the network l0 is balanced. When it is balanced, there is no signal going to the input circuit of amplifier I3; and consequently the output circuit thereof is not energized to call for the operation of the servo motor I6. This condition may be indicated by a voltmeter, lights, or other suitable means, not shown. Under these conditions, switch 380 may be closed and the ailerons will be under the control of the automatic pilot.

The rudder network I I is next balanced. Since the plane is flying with its wings level, wiper |12 of potentiometer I1! in bridge |52 will be centered with respect to resistor |13; and since mechanical linkage 352 has previously been centered, wiper |25 of potentiometer |65 in bridge 5| is centered with respect to resistor |61. As previously mentioned, conductor 9| is grounded through switch 93, thereby placing conductors Sl and 88 of adjusting network 82, wiper 84 of potentiometer 82, and conductor |84 and wiper |54 of the rebalancing potentiometer |53 at ground potential. The rudder has been trimmed by the manual trim tab so that it maintains the plane in the desired heading, and this position is transmitted by the cable to the rudder servomotor I1 which in turn positions the wiper |54 of the potentiometer |53. Wiper |54 may be electrically centered so as to balance the bridge |5| by adjusting the position of the wipers |51 and |6| of the potentiometers |56` and |50. When this has been done, no voltage appears between conductor |24 and ground, no voltage appears between wipers |54 and |63 of the bridge |5|, no voltage appears between conductor |10 and. wiper in the network |52, and hence there is no voltage signal transmitted to the input of amplifier I4. There is thus no output signal from the amplifier |4 tending to cause the motor l1 to operate, which condition may be indicated by lights, a voltmeter, or other suitable means (not shown); and hence the switch 38| may be closed and the ailerons and rudder are now under the control of the autopilot. The mechanical linkage 352 is now released so that the azimuth responsive instrument 50 controls potentiometers 45 and |65.

The elevator network |2 remains to be balanced; and since the aircraft is flying with its wings level, mechanical linkage 35| has centered wiper 22| of potentiometer 220 with respect to its resistor 222. In addition, since the plane is neither diving nor climbing, mechanical linkage 350, which is operated by the vertical gyroscope 40, has substantially centered wiper 2It` of potentiometer 2|5 with respect to resistor 2I1. When switch 9i is in position 2, transformer ||4 is not energized, and thus no voltage appears between grounded wiper 232 and conductor 265, and hence no voltage appears between wiper 204 of rebalancing potentiometer 203 in bridge 20| and ground. In order to maintain the aircraft in level flight, tie elevator has been positioned by the use of a mechanical trim tab so that the craft neither dives nor climbs; and this position is transmitted by the cable to the elevator servomotor |23 which in turn positions the wiper 204 of the potentiometer 203. If this position results in an electrical unbalance of the bridge 20|, the balance may be restored by adjusting the position of wipers 251 and 2|| of potentiometers 205 and 2 I0 until no voltage is obtained between wipers 234 and 2|5. When this is done, since there is no voltage between wiper 204 and ground, there will be no voltage between wiper 2|6 and ground; and there will similarly be no voltage appearing between wiper 23| and wiper 2|5 and hence no voltage between wiper 23| and ground. There will thus be no voltage signal applied to the input circuit of amplifier i5, and the output circuit thereof will not be energized, which condition may be indicated by a voltmeter, lights, or other suitable means (not shown) and hence switch 332 may be closed and the ailerons, rudder, and elevator of the aircraft are now controlled by the autopilot.

With the aircraft flying under the complete control of the autopilot, any change in the angular position of the craft about any of the three axes is immediately detected and compensated for. Thus, if one wing of the arcraft drops, this change in the angular position of the craft will be detected by the vertical gyroscope 4E) which will cause the linkage 35| to move wipers 21,

|12, and 22| of potentiometers 26, |1I, and 220,'

respectively. When this is done, a voltage will appear between wiper 21 and wiper 24 of bridge 2| whose phase will be determined by the direction wiper 2l is displaced relative to wiper 24; and this voltage will be transmitted through network 22 to amplifier I3, where it will cause the output circuit to be energized to operate the aileron servomotor I5 in a direction determined by the phase of the input signal which is in turn determined by the direction of movement of wiper 21. When the servomotor i6 is operated, it will reposition the ailerons and they will exert a force upon the aircraft tending to rotate it about its roll axis and return it to its original position. When the servo-motor I3 is operated, it drives the wiper 24 of the potentiometer 23 in a direction tending to balance the bridge 2|; and when this position of balance is reached, no further signal is transmitted to the input circuit of the amplifier I3, and the output circuits thereof are thus no longer energized and the servomotor i3 ceases rotation. The ailerons, however, are in a displaced position, and as long as they remain displaced from their original position, the aircraft will tend to turn about its roll axis. As the aircraft returns toward its original position, however, wiper 21 of potentiometer 26 is returned towards its center posiampia 19 tion b`y the action of the vertical lgyroscope 4Q and the mechanical linkage 35|. As this occurs, the bridge 2| will be unbalanced in the opposite direction; and the signal sent to the amplifier I3 will thus cause the output circuits to energize the motor I6 so that it rotates in the opposite direction, thereby returning the wiper 24 towards its original position and simultaneously returning 'the ailerons to their original position.

When an airplane is flying with its wings level, the lift produced by these wings is substantially perpendicular to them, and hence this lift is directed vertically upward. When the plane banks or rolls about its roll axis, however, the wings are no longer level, and the lift produced by them is no longer vertical but is directed upwardly and at an angle to the vertical. This means that the vertical component is reduced, and in addition a horizontal component is introduced which tends to turn the aircraft toward the wing which is low.

To compensate for this tendency of the aircraft t'o turn when one wing is low, potentiometer |1| is Vincluded Vin the ruddernetwork I and is so connected that when wiper |12 thereof is moved across resistor |13 in a direction determined by and in response to a movement of the aircraft about its roll axis, a signal whose phase is determined by the direction of movement of wiper |12 is transmitted to potentiometer |14 and thence to amplifier I4 to operate the rudder servo i motor |1 in a direction tending to oppose the turning of the ship caused by one wing being low. When the servo motor |1 is operated, it moves wiper |54 of potentiometer |53 and thereby causes a voltage to appear between it and wiper |66 of bridge |5I. This voltage is in such a direction or phase relationship with the voltage appearing between conductor and wiper |15 as to oppose and cancel the latter; and when these Atwo voltages are equal, no input signal is received by amplifier |4, and the operation of servo motor I1 ceases. As the aircraft returns to its original position with its wings level, wiper |12 is moved towards its original position by mechanical linkage 35| Vand the voltage appearing between conductor |16 and Wiper |15 is thereby reduced. When this happens, the voltage appearing between wiper |54 and wiper |66 is larger; and since it is of the opposite phase to that appearing between wiper and conductor |10, the'amplier I4 is energized so as to rotate the Yservo motor i1 in the opposite direction, thereby returning the rudder towards its original position and likewise returning wiper |54 towards its original position.

As previously mentioned, when the aircraft banks about its roll axis, the Yvertical component of the lift produced by the wings is reduced,and the plane therefore tends to lose altitude. This, of course, is very undesirable; and to overcome this tendency of the aircraft, wiper 22| of the potentiometer 22|) in the elevator bridge 262 is moved by the mechanical linkage whenever the plane is banked. As previously mentioned, the ends of resistor 222 of potentiometer 220 are connected together by conductor 223, and thus whenever wiper 22| is displaced from the center top by the movement of linkage 35|, a potential difference is produced between that wiper and conductor 226 to which the center tap is con- A nected. Because the ends of the resistor 222 are connected together, this voltage will always be of a certain definite phase; and this phasing has been selected so that the voltage produced besistor is 'such as to cause the amplifier 'I5 to op erate the servo 4motor i6 in a direction to move the elevator upwardly and increase the 'lift of `the airplane. With this construction, it will be seen that whether the plane banks about its roll axis to either the left or the right, the signal produced by potentiometer 226 will be of the same phase and the elevator will be moved upwardly in either event.

When the elevator is moved upwardly, wiper 204 is moved by servo motor |`8 to a.` new position so that the voltage appearing rbetween wiper'264 and wiper 2|6 will be equal and opposite to that appearing between wiper 23| and conductor 226, whereupon operation of the servo motor |8 will cease. As the plane returns to its original 'position 'with its wings level, the mechanical linkage 35| will return the wiper 22| to center; and since the voltage appearing between wipers l2634 and 2|6 will now be larger than that appearing between wiper 23| and conductor 226, the amplifier I5 will operate the servo motor I8 in the opposite 'direction to return the elevator to its original position.

It will thus be seen that when the plane is acted vupon by air currents to cause one wing to drop, the Vailerons are moved so as to restore the wings to their level position, the 'rudder is moved to prevent the turning ofthe airplane, and at the same time a 'certain amount of up elevator is applied to maintain the altitude of the plane. "As theplane returns to its level position, the ailerons are returned to their original position 'andthe rudder vis likewise returned, while at the 'same time the amount of up elevator which was introduced during the bank is removed and the elevator returned to its original position.

Let it now be assumed that the aircraft is turned in azimuth by the action of air currents, while at the same time the wings are maintained in their original level position. Under these oonditions, the azimuth responsive instrument 5|) will Ysense this and move the mechanical linkage 352 so that wiper 41'of potentiometer 46 in the aileron bridge 22 and wiper |66 of potentiometer in rudder bridge |5| will be moved across their respective resistors and cause signals to be transmitted to amplifiers |3 and i4, respectively.

Since the heading of the aircraft in azimuth isV to be changed, the need for operation of the'rudder will be apparent; and the movement of wiper |66 across resistor |61 of potentiometer |65 will cause a voltage to appear between wiper |66 and |54 which will be transmitted to the rudder amplifier I4. This will cause the rudder servo motor |1 to be operated to reposition the rudder in a direction tending to return the plane toward its original heading, and wiper |54 will simultaneously be moved until the balance of the bridge is restored.

The need for operation of the ailerons when the aircraft is to be turned in azimuth is not so apparent as the need for operation of the rudder as just described. However, for the quickest return of the aircraft to its original position, the turn must be coordinated so that the resultant of the centrifugal and gravitational forces acting upon the'aircraft during the turn will pass through the craft inthe same direction that the gravitational force alone does when the plane is flying with its wings level. To insure that the turn made by the plane under these conditions is coordinated, wiper 41 of potentiometer 46 lis moved by the mechanical linkage 352, as previously mentioned, whenever the azimuth -responsive instrument 50 indicates that a deviation has been made from a desired heading in azimuth. As a result, a signal appears between wiper 41 and the center tap of resistor 48; and the amount of this signal which is transmitted to amplifier I3 may be Varied by adjusting wiper 56 of potentiometer 55. When the signal is applied to amplifier I3, of course, it operates the servo motor I6 to reposition the ailerons the necessary amount to provide the desired degree of bank.

From previous discussions, it will be seen that whenever the plane banks about its roll axis, a signal is put into the aileron and rudder networks I and II, respectively, by the vertical gyroscope 40; and thus when the plane is caused to bank and make its coordinated return to its original heading in azimuth, the vertical gyroscope causes the mechanical linkage 35| to reposition wipers 21 and |12 of potentiometers 26 and |1| in the aileron bridge and rudder networks, respectively. The effect of this is to decrease the signal sent to the aileron and rudder amplifiers I3 and I4, or expressed differently, the deflection of the ailerons and the rudder is reduced. The magnitude of the signals reaching the amplifiers I3 and I4 from networks 22 and |52 is determined by the position of the wiper 56 of potentiometer 55, and Wiper |12 of potentiometer |1|. By varying the amount of this opposing signal from the potentiometer |1I, operated by the vertical gyroscope 40, the resulting effect of the displacement of the wiper |66 may be controlled. Similarly, by varying the signal from the potentiometer 46, controlled by the azimuth responsive instrument 50, the amount of signal from the potentiometer 25, controlled by the vertical gyroscope 40, necessary to cancel the rst signal may be controlled. As alresult, the angle of bank for a given deviation in azimuth may be controlled by the adjustment of potentiometer 55. In the same way, the deflection of the rudder may be controlled by the adjustment of potentiometer |14. To adjust the potentiometers, the plane will generally be placed in a turn by displacing the mechanical linkage 352 to one side and holding it in that displaced position while the necessary adjustments are made. The wiper 56 is adjusted until the plane is banking at the desired angle for the displacement of linkage 352, and. wiper is then adjusted until the turn and bank indicator on the instrument panel indicates that a coordinated turn is being made. Under these conditions, the effect of the azimuth responsive instrument and the effect of the vertical gyroscope 40 are both present in the networks l0 and II.

After the necessary adjustments have been made, the mechanical linkage 352 is returned to center position, and any further movement thereof will first cause the ailerons and rudder to be displaced, and then as the plane banks, the control surfaces will be returned toward their original position as previously mentioned.

At the same time that the plane starts to bank about its roll axis, and the wipers |12 and 21 of potentiometers |1| and 26, respectively, are moved, wiper 22| of potentiometer 220 is also moved to provide an upward movement of the elevator to maintain the altitude of the plane,

as previously described.

Should the plane tilt about its pitch axis, i. e., if the nose of the plane is raised or lowered, this change in the angular position of the aircraft will be detected by the vertical gyroscope 40 and will cause the mechanical linkage 350 to be moved and the latter will reposition wiper 2I6 of potentiometer 2|5 in the elevator bridge 20|.

When this is done, a signal will be transmitted 5 to the elevator amplifier I5 which will cause the latter to operate the servo motor I8 in a direction tending to restore the desired attitude of the plane as measured about its pitch axis. As the motor I8 is operated, the wiper 224 of the potentiometer 203 will be repositioned until a condition of balance is obtained for the bridge at which time operation of the servo motor I8 will cease. When the plane is once again flying as desired, the vertical gyroscope 40 will cause the mechanical linkage 350 to return the wiper 2 I6 to its original position, thereby unbalancing the bridge 20| in the opposite direction and causing the amplier I5 to operate the servo motor to again reposition the elevator and return it to its original position. In this way, the altitude of the aircraft is generally maintained whenever any change of the craft about its pitch axis tends to change the altitude thereof.

If the aircraft is flying at a substantially constant altitude, as previously described, and it is desired to change that altitude, it is only necessary to reposition wipers 201 and 2I| of potentiometers 266 and 2|0 in the proper direction. When this is done, the bridge 26| will vbe unbalanced and the signal appearing between wipers 204 and 2i6 will be transmitted to the amplifier I5 and the latter will cause the servomotor I8 to operate to reposition the elevator and to move wiper 254 toward a position of balance for` the bridge 20|. As the elevator starts to move, the angle of the plane about its pitch axis will be changed; and'this will cause the linkage 350 to be moved by the vertical gyroscope 40 and reposition wiper 2 I 6 of potentiometer 2| 5. The movement of wiper 2|5 is in a direction tending to balance the bridge with the wiper 204 displaced from its original position, and as a result, the elevator will remain somewhat displaced from its original position until the wipers 201 and 2|| of potentiometers 206 and 2I0 are returned to their original positions.

If, because of movement of passengers or other shifting of load, the center of gravity of the plane is shifted along its length, the original position of the elevator will no longer be suitable to maintain level flight. Under these conditions, the potentiometers 255 and 2I0 may be adjusted to reposition the elevators and return the craft to level flight. The action of the amplifier I5, servomotor I8, potentiometer 203, vertical gyroscope 40, and potentiometer 2|5 is similar to that previously described in connection with the change of altitude of the craft.

If, because of changes in loading or changes in 30 power, one wing of the aircraft tends to drop below the other, the potentiometers 34 and 33 may be readjusted to change the normal position of the ailerons and thus restore the craft to its former angular position about its roll axis. Similarly, the normal position of the rudder may be changed by adjusting the potentiometers |56 and |60 of the rudder bridge I5|; and the operation of the aileron circuit and rudder circuits under these conditions is similar to that of the 7o elevator circuit previously described.

With the switches 93 to 91, inclusive, in position No. 2, movement of the controllers I|1 and |21 has no effect upon the operation of the aircraft. When the switches are moved to position 3, however, transformer ||4 is energized through switch 91 and conductor-9| -isconnected through switch '9'3 to -movable contact w. If -it is now assumed that transfer relay operating coil lis deenerg'ized and all the movable contacts of that relay are bearing against their corresponding out contacts, conductor Si isconnected tocons ductor 'IG-1 leading to network |08. VSince transformer M is -now energized, secondary winding ||3 thereof 'impresses a voltage across the ends of V'resistor H2 of Vpotentiometer H0; and any movement of vgrounded wiper ||l| away from the center tap of that resistor willfcause a voltage to Vappear Ibetween conductor |01 and ground. This `voltage will .be transmitted through fout contact |06 and `movable contact |04, through conductor |113, swi-tch 93, and conductor 9| to the'adju'sting network 3E, where it will be impressedacross resistors-S5 and of :potentiometers -n8| Yand E2, respectively. These resistors with `Vtheir associated fwipers, act as voltage dividers and the desired `portieri of the voltageappearing between conductor |51 and ground may be selected by adjusting 'thelwipers to the proper position.

From rwiper 83, the voltage is transmitted as previously described to the input of amplifier i3. This lsignal causes the vamplifier to operate A.the servomotor to reposition 'theailerons and Ysimultaneoslytomove the ywiper 2li of potentiometer 23. The directionof movement of Vwiper 213 is such ras 'to Ycause asignalto Yappear between that ywiper and rwiper -21 of potentiometer 2&3 which `is equal -in magnitude and opposite in phase to that appearing between wiper 83 fand ground. As the ailerons start to move, the'plane starts lto move about its roll axis; and when the signal appearing between -wipers-2|| and-21 cancels that appearing between wiper 83 and'ground, the operation of ythe vservomotor I6 ceases. As the plane changes its angle about its roll axis, however, the vertical gyroscope d'0 senses this change and causes the mechanical linkage 35| to move wiper'21 of potentiometer 26. Sincethe plane is tilted inone direction, the verticallgyroscope will move the potentiometer wiper 2.1 infa direction to cause a signal to be put in which would normally drive the-ailerons ina direction to return the aircraft rto a position where its wings are level. However, because `of thesignal put inby bridge |68, the ailerons are not driven in the opposite direction but are merely returned to their original position so that banking of the plane Vabout its roll axis does not continue-"but instead the plane remains in the bank it vhas then assumed.

At this time, the controller ||1 is being" held to one side, with a wiper moved across the resistor ||2. The plane is in a bank and the vertical gyroscopell has caused the potentiometer wiper A21 to be displaced along the resistor 28, and the wiper 24 has been returned to its original position. The Vvoltage vappearing ubeftwe'en'wip'ers 24 and 21 is equaliinmagnitude and opposite in phase to that appearing .between wiper 83 and `ground, and there is thus no Lsignal being transmitted to the amplifier |13 to operate the servomotor |5.

When the control device ||1 is returned to its center position, wiper l|| is likewise returned to its center position, and the voltagepreviouslyfimlpressed across resistor `85 in network '80 is removed and wiper 83 `is thereby returned to ground potential. When this happens, the onlyvoltage l'remaining in the'aileron circuitisthat-appearing between -lwipers 24 fand 21 `in the bridge "-2 I, 4and this Lis phased so `that when 4it is limpressedlupon the amplifier i3, the ilatter operates theservo motor I6 in a direction to fcause the ailerons V'to return the plane Yto the position where its wlings are level. VThis operation is similar `to that previously described for the condition when-chewing was caused to ldrop by the action of aircurrents, and will not be repeated here.

The Voltage which appears between conductors 81 and-8S, when wiper V| is displaced 'fromits center position to produce a voltage between-Wiper 33 and ground, also Vproducesa voltage between wiper Se and ground. This voltage is conducted by conductor |813 lto wiper ld of bridge i|f5`| and is transmitted from that bridge through network |52 and then to the input of amplier |4. #Ampli- Iier E4 then energizes servo motor |1ltofcause1the latter to reposition the -rudderso as tocause'the plane to turn in a direction toward the Vwing which is lower. .As the rudder is -repositioned, wiper |55. of potentiometer l|53 ismoved in a direction to cause a signal to appear between-:that wiper and wiper it which-is equal in magnitude andiopposite in phase to thesignalappearing-between wiper-3:3 and ground. YThe-voltage signal to the arnpler i4 is thereby canceled vand vr`the operation of the servo motor |1 ceases.

At the same timethat therudder is repositioned to cause the plane to turn,'theailerons are1repositioned to place the plane in a bank-asfpreviously described; and wiper |121of potentiometer |1f| lis thereby displacedby mechanical vlinkage 35 I-acting under the influence of the vertical gyroscope 46. The voltage appearing between wiper |12 and the center tap of resistor |13 is applied tothe ends of resistor |16 of potentiometer |14; and-aportion of this voltage appears between wiper |151and conductor i1 and is transmitted to the'amplier |t. This latter voltage will oppose the voltage appearing 4between wiper 84 and ground; and-as the plane banks, the rudder will therefore bereturned towards its original position after `-having been displaced therefrom by the operation Vof potentiometer Ht and bridge |88. In "this VV`way, the rudder is first deflected a Vlarger `amount to place the craft in a turn, and then as rvthe plane banks in that turn, the rudder de'ectioneisireduced to a point just 'sufcient to produce acoordinated turn.

When the wiper Yof thepotentiometer I0 is returned to itscenter position, conductorST-is returned to ground potential Yas lis wiperdxconductor |84, and wiper Bt. The voltage appear'- ing between Awiper |15 of potentiometer I'Ul in network |52 is considerably larger than lthe voltage appearing Abetween wiper |66 Aand wiper I5@ of bridge |5|; and the Vvoltage from network |52 therefore dominates and causes the amplifier Hi to energize servo motor |1n a direction vto return `the rudder and wiper |55 lto their original positions. At the `same time this ishappening, the ailerons are returning the plane Vto a position with its wings level, and vertica1 vgyroscope 4U is therefore moving mechanical linkage 35|to'return wiper |12 to it'soriginal position, and'there by returning the plane Aand the control networks -totheir original condition.

litsoriginal position, the aileron and rudderfare thenoperated to cause-the plane to straighten' out and fly with its wings level on the new heading. Furthermore, it is to be understood that while the operation of the various potentiometers has been described in sequence, in actual operation these various movements and controls occur simultaneously instead of step by step.

When the aircraft is banked and turned under the influence of the control device I1, the change of the angular position of the craft about its roll axis causes the vertical gyroscope 40 and the mechanical linkage 35| associated therewith to move the wiper 22| of the up-elevator potentiometer 220 in a manner previously described to apply up-elevator and maintain the plane at substantially the same altitude. Aside from the application of up-elevator when the plane banks, the movement of wiper across resistor ||2 causes no action in the elevator circuit of the control system.

When the airplane is being turned by means of the control device |I1 as just described, the normal operation of the azimuth responsive instrument 50 must be interrupted so that it will not introduce signals into the control network which would tend to nullify the action just described. Even if it were possible to overcome the action of the azimuth responsive instrument to a certain extent by the operation of the control device, when the new heading had been achieved and the control device were returned to its center position, the azimuth responsive instrument would have displaced the mechanical linkage 352 and would then return the plane to its original heading in the manner previously described in connection with an accidental change in azimuth. To prevent this from occurring, the switch 35D is operated whenever the wiper is displaced from its center position. Contact 353 of switch 360 is energized whenever switch 95 is in position No. 3; and whenever the wiper is displaced from its center position, the movable finger 38| permits contact 302 to bear against contact 303 and thereby complete a circuit through out contact 31|, movable contact 312, and conductor 313 to the locking means 353 associated with the azimuth responsive instrument 50. Similarly, when switches 93 to 91 are in position l, the locking means 353 is energized through conductor 355, movable contact 354 and in contact 350 of servo relay 59, conductor |3|, switch 04, conductor 330, and conductor 325, as previously described. The locking means 353 will thus be energized when switches 03 to 91 are in position E, or when the switches are in position 3 and the operating control device ||1 or |21 is displaced from normal position. The need for the energization of the locking means 353 when switches S3 to 91 are in position will be hereinafter explained. Linkage 352 is thus held in its centered position; and the azimuth responsive instrument 50 slips with respect to the mechanical linkage 352, and there is thus no movement of wiper 41 of potentiometer 46 in network 22, nor of wiper |06 of potentiometer |55 in bridge |5|.

When the control device ||1 is moved so that grounded wiper 262 is moved across resistor 263 of potentiometer 20|, a voltage is produced between that wiper and conductor 255 which is transmitted through out contact 265, movable contact 210, resistor 21|, and conductor 212 to Wiper 204. From Wiper 204 this voltage signal is transmitted through bridge 20| and network 202 to the input terminals oi amplifier I5, and the latter causesthe servo motor I3 to be operated to reposition the elevator and wiper 204 of potentiometer 203. As wiper 204 is moved, a voltage appears between it and wiper 2|6 which is opposite in phase and increases in magnitude until it completely cancels the voltage appearing between conductor 205 and ground. When this occurs, the operation of the servo motor I8 ceases. The elevator is thus deflected and the airplane starts to climb or dive depending upon which way the elevator is deflected. The climbing or diving is caused by the change of the angular position of the aircraft about its pitch axis, and as this position is changed, the Vertical gyroscope 40 senses the change and causes the mechanical linkage 350 to move the wiper 2| 6 of the potentiometer 2|5. This introduces a signal having a phase opposite to that of the signal from the network 250 and as a result the amplifier l5 causes the servo motor I8 to drive in such a direction as to return the elevator toward its original position. The aircraft will therefore continue to climb or dive at a rate determined by the position of the control device ||1 and by the air speed of the craft. When wiper 282 of potentiometer 26| is returned to its center position, the voltage appearing between conductor 265 and ground is removed, and wiper 204 is thus returned to ground potential. A voltage thus appears between wiper 204 and wiper 2|6 which has an opposite phase to that originally appearing between conductor 255 and ground. This voltage is applied to the input terminals of amplifier 5 and causes the servo motor |8 to be operated in the reverse direction, thereby reversing the deflection of the elevator and moving wiper 204 at the same time. This action of the elevator causes the plane to level out, and the vertical gyroscope 40 thereupon causes the mechanical linkage 350 to return the wiper 2|6 to its original position, causing the elevator to be returned once more to its original position as the wiper 204 also is moved by servo motor |8 in 'response to signals to the amplifier |5. In the elevator network 2, as in the aileron and rudder networks l0 and the movement of the rebalancing potentiometer wiper 204 and the wiper 2|6 of the potentiometer 2|5 has been described as sequential acts; but it is to be understood that this is for the purpose of description only, since these movements occur substantially simultaneously instead of sequentially.

The control device ||1 is designed so that potentiometer l0 and potentiometer 262 may be simultaneously and independently controlled; and it is thus possible to operate potentiometer H0 without operating potentiometer 26|, potentiometer 25| may be operated without operating potentiometer 0, or both potentiometers ||0 and 25| may be simultaneously operated. If potentiometer H0 is operated, the plane will be turned; if potentiometer 23| is operated, the plane will be caused to climb or dive; and if both are operated at the same time, the plane will be caused to make a climbing or diving turn.

It is sometimes desirable to have the control device H1 operative to control only the elevator of the aircraft and inoperative to control the ailerons and rudder thereof, so that if the control device is accidentally moved in a manner to operate the potentiometer ||0, there Will be no effect upon the ailerons and rudder. Such a condition may arise when it is desired to make a series of photographs of a certain strip of terrain and it is necessary that the altitude be maintained very accurately in spite of updrafts or `downdrafts of air. Under these conditions, the

. 2? autopilot is allowed to control the heading of the ship in azimuth so that the desired constant course is flown, while the pilot maintains the altitude of the plane by means of the controller l il and the elevator controls.

To` accomplish this, the ganged switches 93, 94', 95', 95, and Si are turned to position di. When this is done, conductor Si is grounded through switch 93v at Q9, and any voltage appearing between conductor lill and ground is impressed. across resistor H5. Since conductor ill is maintained at ground potential, conductor Sl' and hence wipers 83 and 8d of potentiometers di and 32 are likewise maintained at ground potential; and any movement of grounded wiper lil away from its center position will have no eifect upon either the aileron network lil or the rudder network l l. The operation of the bridge circuits I and ll is thus the same as when the switches 93 to 9i, inclusive, are in position 2, and any tiltn ing of the wings or turning of the plane in azimuth is sensed by the vertical gyroscope lil andthe azimuth responsive instrument 5% and immediately corrected. Switch blade 3&5 of switch 95 is in position d, butin this position there isvnopower applied to the switch blade, and hence switch 3S@ is ineffective to operate the brake or locking mechanism 353 of the azimuth responsive instrument Eil.

Conductor 265, however, leading from network 260, is connected through out contact 26S and movable contact 2li! of the transfer relay HifiA through resistor 2li, conductor 222, resistor 291, and switch 95 to ground at 2st. Any voltage appearing between conductor 2555 and ground, because of the displacement of grounded wiper 252 from its center position, will be impressed across resistors 2l'l and 294 in series. These resistors together form a voltage divider, and their common point of connection will have adenite potential with respect to ground which will be a constant fraction of the voltage appearing between movable contact 2li) and ground. Wiper 2M of rebalancing potentiometer 203 in the elevator bridge 28d is connected to conductor` 212', one of the conductors connecting resistors 2li and 29! together. As a result, wiper 2li-i will. always be at a potential with respect to ground which is a fixed percentage of the voltage appearing between conductor 265 and ground. The voltage appearing between wiper 204 andv ground will be transmitted to bridge Zi and network 222 to the elevator amplifier l5, Where it will cause the latter to operate the servo motor I8 to reposition the elevator. The balancing operation of the network I2 under these conditions is in all respects similar to that previously described in connection with the operation or" the elevator network l2 when the switches 93 to 91, inclusive, are in position 3. However, because of the voltage dividing action of resistors 21| and 29E, the movement of the elevator for a given movement of the grounded wiper 252 will be materially reduced when the ganged switches 93 to 91, inclusive, are in position ai instead of position 3. This is a very desirable effect, since under the conditions when these switches are in position il, a delicate, Vernier action of the controller lll is desired rather than a coarser control with a greater amount of control.

It wil be seen that when switches 23 to Si, inclusive, are in positions 2, or s, the responsive instruments Il@ and 5i! are eective to control the flight of the aircraft. However, when the responsive instruments lid and 5B are effective to control the flight off the aircraft, their action is generally to reduce the'deiiection of the control surfaces as the aircraft reaches the desired new angular position. While the speed of change of the angularl position of the aircraft is generally suicient for all practical purposes, generally being suiiiciently great so that the passengers may be made uncomfortable should such speed be necessary, under certain emergency conditions it is necessary to exceed even this rate of change of angular position.

To secure the advantages of servo motor control of the control surfaces, while at the same time avoiding the slower response thereof caused. by the action of the responsive devices l2 and 5l), and also avoiding the limitations of the angular deviation of the craft imposed by the responsive devices, the switches 93 to Sl, inclusive, may be turned to position No. 1. When this is done, the servo relay 59 has its operating coil 62 energized from the source of power 32%, through conductors 325 and 330, switch 9d, and conductor i3! to the coil 62 and from the latter to ground and back to the source of power. When the operating coil 62 is energized, the movable contacts 63, |82, 2371, and 35d are moved from out contacts Si, i, 235, to in contacts i3, liiljZ, and When contact 255i bears against in contact dit, a circuit is completed 'from condoctor lei, which is connected through switch as previously mentioned, through conductor del to the locking means 353 and then to ground. If wipers :il and of potentiometers is and 55 are centered, as they should be when switches S3 to Si, inclusive, are moved to position l, the operation of the locking means 3.53 will hold these wipers in their centered positions, regardless of the change in heading of the aircraft; and there wil thus be no signal introduced into the aileron network il! or the rudder network ll because of the operation of these potentiometers. The movement of the mechanical linkages 35) and controlled by the vertical gyroscope All, will .1 no way be restrained; but, as will he explained, the operation of the potentiometers controlled by these linkages will have no eect upon the input circuits of the ampliers I3, ill, and l5.

When switches Q3 to il?, inclusive, are placed in position l, transformer lill, which energizes the bridges Hi8, Zilli, E22, and 25E, will be-energized by the inverter 3il3 through switch 91. Simultaneousl, since contact lil! oi switch 93 is not connected to anything, conductor @l is connected to network H33 through resistor l ifi, movable contact Hill, and out contact it.

In general, the current drawn through the networks lil, ll, and i2 is quite small, since the input circuits in the amplifiers i3, ld, andl are connected to the grid circuits of amplifier tubes. As is well known, the current drawn by the grid circuit of an amplier tube is very small, and consequently the voltage drop through a series resistor in the various networks is generally considered to be negligible. In the case of adjusting network 80, two resistors, 35 and B6, are connected in parallel and are connected to act as voltage dividers, as previously mentioned. Such a circuit is a very satisfactory method of reducing a voltage which is to be applied to a vacuum rtube control element. To reduce still further the voltage appearing between wiper 83 and ground and wiper Esi and ground. resistor ll is connected between the source of voltage, network IBS, and network 80.

It will be seen that for a given deflection of vgrounded wiper I|I from its center position, a voltage will appear between conductor |01 and ground, and when switch 93 is in position 3, the full value of this voltage will be transmitted through conductor 9| to conductor 81 where it will be impressed across resistors 83 and 85. However, when switch 93 is in position No. I, a series parallel circuit will be formed which includes resistor IIB in series with the parallel resistors 85 and 86. As a result, the voltage appearing between conductor 81 and ground will be materially reduced in the manner of the well known voltage divider; and likewise the voltage appearing between wiper 83 and ground and wiper 84 and ground will be much smaller when switch 93 is in position I than when it is in position 3. Consequently, the signal appearing between wiper 24 and ground and wiper |54 and ground will be much smallerwhen switch 93 is in position I. The need for this reduced signal 'whenswitches 93 to 91, inclusive, are in position I, will be explained hereinafter.

As previously explained, when switches 93 to 91, inclusive, are in position I, the circuit in the aileron network I is traced from wiper 24 through potentiometer 23, potentiometers 33 and 34, and potentiometer 10 to the in contact 13 of the servo relay 59, and then through movable contact 63 to the input of amplier i3. Potentiometer 10 is a manually adjustable potentiometer which is provided to complete a bridge circuit including the rebalancing potentiometer 23 and the secondary winding 32. The potentiometer 1|I is adjusted so that when switches 93 to 91 are moved from position 2 to position I,

night, with the switches 93 to 91 inclusive, in

position 2, 3, or 4, and the switches are then moved to position I, no voltage will appear between wiper 1I of potentiometer 19 and wiper 24 of rebalancing potentiometer 23. If the control device ||1 is now moved so that grounded wiper III of potentiometer IIO is displaced from its center position, a voltage will appear between conductor |91 and ground, and a portion of this voltage will appear between wiper 24 and ground. This portion of the voltage will be transmitted to "in Contact 13 of the servo relay 59 and then to the amplifier I3, where it will energize the latter to cause the operation of the servo motor I6 to reposition the ailerons and simultaneously move the wiper 24 to a new position where in the voltage appearing between wiper 24 and wiper 1| will be equal and opposite to that appearing between wiper 24 and ground. When this occurs, the voltage signal applied to the input terminals of amplier I5 will be reduced to zero, and the operation of the servo motor I 6 will cease.

The ailerons of the aircraft are now displaced, and they will remain in this displaced position until wiper III of potentiometer IIB is returned to center position. The displacement of the ailerons will cause the aircraft to be angularly moved about its roll axis, continuing this rotation about its roll axis as long as the ailerons are displaced.

When the control device ||1 is returned to its center position, wiper I is likewise returned to its center or normal position, and wiper 24 is returned to ground potential. The only voltage `now transmitted to the amplifier input is that Ywhich appears between wiper 24 and wiper 1i,

and this voltage is such as to cause the servo motor I6 to be rotated so as to return the ailerons to their original position and reduce the voltage appearing between wiper 24 and wiper 1| to zero. This means that to cause the airplane to rotate a given amount about its roll axis, the control device II1 must be displaced to one side to cause the ailerons to be deilected; and then as the aircraft approaches the desired angular position, the control device I|1 is then returned to center. The aircraft will tend to remain in this banked condition until the ailerons are deflected in the opposite direction to return the plane to a condition of flight with its wings level. To do this, the control device I i1 is moved in the opposite direction to cause the ailerons to be deflected in a direction opposite to their first deilection, and then as the plane approaches a position with its wings level, the controlled device is returned to center position.

It will be seen that this operation of the control device II1 is very similar to that of .the well known stick used in the control of aircraft. where the stick is moved to one side to deflect the ailerons and cause the plane to deviate about its roll axis, and the stick is then neutralized or centered to return the ailerons to their original position Vand prevent continued rotation of the plane about its roll axis. To return the plane to a condition of flight with its wings level, the stick is displaced in the opposite direction; and as the aircraft returns to its original position, the stick is again neutralized. This action is quite diierent from that obtained when the switches 93 to 91, inclusive, are in positions 3 or 4, where the degree of angular movement of the plane about its roll axis is determined by the amount that the control device |I1 is moved from its center position. As the aircraft appreaches this position with switches 93 to 91 in positions 3 or 4, the action of the vertical gyroscope 49 is such as to neutralize the controls or return the ailerons to their original position, thereby preventing continued rotation of the aircraft about its roll axis.

Since the balancing eifect of the gyroscopically operated potentiometer 26 is removed from the aileron network IIl when the switches 93 to 91 are in position No. I, the voltage impressed across the input terminals of amplifier I3 would be considerably greater for a given movement of the control device II1, when switches 93 to 91 are in position I than when they are in position 3. The lack of a balancing voltage from the potentiometers controlled by the responsive devices 49 and 50 may permit the servo motors i6, I1. and I8 to be driven to the limits of their` travel with very little movement of the control device I|1, thereby providing a very coarse control. It is for this reason that the resistor I I6 is inserted in series with the adjusting network 89 when the switches 93 to 91, inclusive, are in position No. I.

Somewhat similar action is obtained from the rudder network when switches 93 to 91, inclusive, are in position No. I. As previously mentioned, when the switches are in this position, the locking means 353 of the azimuth responsive instrument 5D is energized, holding the mechanical linkage 352 in the centered position in which it has previously been placed. The potentiometer I1I is operated by the mechanical linkage 35| attached to the vertical gyroscope 4D, and it is the operation of this latter potentiometer which must be by-passed in order to provide the desired control characteristic. To do this, .in contact I8I of servo relay 59 is connected by conductor to conductor |10; and movable 31 contact |82 is connected.- by conductor i3@ to wiperl |15, When ser-vo relayV e is energized, conductor |l is connected through relay contactsr |8| and |82 to wiper |15, thereby short-ing the output terminals ofk network |52 and reducing its effect to zero. Themovement of wiper |12 by the mechanical linkage 35| thus has no eiect upon the voltage transmitted to the input terminals of amplifier I4 when switches 93 to 91, inclusive, are in position No. i. Since potentiometer |1| has no effect upon the signal transmitted to the amplifier I4, and since wiper |56 of potentiometer |65 is held against movement by the locking means 353', the eiect of the responsive instruments 4!) and 5G is eliminated when switches 53 to 91 are in position i.

Inview of the foregoing, it will be seen that when control device ||1 is moved to displace wiper from its normal position, a voltage appears between wiper 34 of potentiometer 82 in the adjusting network 8i). This voltage is transmitted by conductor |84 to wiper |54 of potentiometer |53 in the rudder bridge |5|; and from there it is transmitted by conductor |10, conductor |89, to contacts |8| and |82 of thel servo relay 59 and by conductors |83 and |18 to the rudder amplifier I4. When the input circuit of the amplier i4 is so energized, it energizes the servo motor |1 to cause it to drive the rudder in a direction determined by the direction of movement of the control device ||1, and the motor simultaneously repositions the wiper |54 of the potentiometer |53 in the network |5|. A voltage thus appears between wiper |54 and |55 of the bridge circuit, and when this voltage reaches a value where it is equal and opposite to the voltage appearing between wiper |54 and ground, the two voltages cancel each other and the signal transmitted to the amplifier 4 is therefore reduced to zero and operation of the servo motor |1 ceases. Since the responsive instruments 40 and 50 are effectively disconnected from the circuit, the rudder will remain in its displaced condition so long as the control device |1 remains displaced from its normal position. When the controller is again centered, wiper |54 of potentiometer |53 will be returned to ground potential; and the signal impressed upon the input circuit of amplifier |4 will be such as to cause the servo motor |1 to drive the rudder back to its original position, simultaneously returning wiper |54 to its original position.

Considering now the operation of both the aileron and rudder networks ||l and l, respectively, it will be seen that when the control device ||1 is displaced from its center position, a signal will be impressed across the input terminals of amplifiers |3 and I4 causing these amplifiers to operate servo motors I5 and I1, respectively, thereby causing a deflection of the ailerons and rudder.

When this is done, the airplane will turn about direction, thereby causing the ailerons and rudder to be deflected in a direction opposite to their 'iirst deiection, and the aircraft will thus return to a position with its wings level-but withY a new heading in azimuth. As the wings approach the level position, the control device ||1 will be. re.- turned to its center position, thereby again returning the ailerons and rudder to their original positions.

The operation of the elevator controls when switches 93 to 91, inclusive, are in position |4 is similar in most respects to the operation of; the aileron circuit previously described. When the control device ||1 is operated so as to movethe grounded wiper 252 of the potentiometer 26|- away from its normal position, a voltage appearsl between conductor 265 and ground which is transmitted to wiper 254 of potentiometer 203 and bridge 25|. From wiper 2M, the circuit for the signal continues as previously traced to the input circuit of amplifier I5. This voltage signal then causes the latter to operate the servo motor- |8 to reposition the elevator, while at the same time wiper 294 is repositioned so that a voltage appears between it and wiper 25| which' is opposite in phase and increases in magnitude until it completely balances the voltage appearing between conductor 255 and ground. When this balanced conditionis reached, amplifier |5 no longer causes the servo motor I8 to drive, and movement of the elevator ceases. When wiper 262v is returned to its normal position, the voltage appearing be,- tween wiper 264 and ground is removed,` and the voltage appearing between wipers 25| and 2541s such as to cause the amplier |5l to drive the servo motor i8 to restore the elevator to its original position, simultaneously restoring the wiper 254 to its original position.

It will be noted that when switches 93 to 91, inclusive, are in position no provision is made for decreasing the voltage appearing between conductor 265 and ground to a lower value for application to wiper 254 of potentiometer 203. While resistor 2?! is connected in series between wiper B4 and movable contact 21B of the transfer relay, the amountcof current iiowing through this resistor and to the input circuit of the amplifier l5 is so small, as previously mentioned, as to cause a practically negligible voltage drop. In the case of the aileron and rudder networks and a resistor H6 or similar means must be connected in the circuit between the network |58 and the adjusting network 80 because of the fact that a relatively high voltage is impressed across the ends of resistor H2 of potentiometer |15. This higher voltage is necessary because of the fact that the plane is often banked to an angle of 40 degrees with the horizontah and the relatively high voltage is needed to overcome the balancing effect of the potentiometers controlled by the responsive instruments 4E! and 50. In the case of the elevator circuit l2, a much smaller voltage may be impressed across the ends of resistor 263 of potentiometer 25| in network 260, since the aircraft normally does not have its angle about its pitch axis changed to such a marked degree as in the case of the angular change about the roll axis. For this reason, the voltage obtainable between conductorV 265 and ground will be much smaller than that obtainable between conductor |01 and ground; and consequently no special means are necessary to prevent too high a voltage from appearing between wiper 24 and ground.

The entire description heretofore has been predicated upon the basis that coil |05 of the transfer relay |59 is deenergized and that-,the pilot has control of the autopilot throughV his operation of control device and the networks |08 and 260 associated therewith. If the copilot wishes to secure control, he momentarily closes switch 336 to energize coil |05, and when the movable contacts associated therewith are moved to their in position, switch 336 may be opened and the circuit will be maintained through movable contact 338 and in contact 331 of the relay. The other movable contacts |04, 210, and 312 of the transfer relay |09 have been moved from their out to their in positions, and networks |68 and 260 have been disconnected and networks |22 and 286 substituted in their place. The copilot may then control the operation of the aircraft by his operation of control device |21, and the response of the plane is in all respects identical to that previously described for the condition when the pilot had control. When the pilot wishes to regain control of the airplane, he merely closes switch 346 momentarily, thereby shorting coil and deenergizing it, whereupon the movable contacts of the transfer relay |09 will be moved to their out position and control device will once again be effective to control the flight of the aircraft.

It will thus be seen that we have provided a control system in which the pilot may have the choice of any one of several different types of operation. He may use the autopilot alone to provide a smooth comfortable flight following a precise path; he may use the autopilot and modify its action by the control device he may use the autopilot and modify its action in one axis only by using the control device or he may effectively disconnect the gyroscopic or other suitable control of the autopilot and use only the servo motors to control the position of the control surfaces as determined by the operation of the control device Ill. In this last form of operation, the control device is operated as the conventional stick in an airplane, but the pilot is relieved of substantially all of the physical labor of flying under these conditions. By the use of this system, pilot fatigue is greatly reduced, and the possibility of pilot-caused errors due to fatigue is materially reduced.

While we have shown and described a preferred form of our invention, we do not wish to be limited to the particular form or arrangement of parts herein described and shown, except as covered by our claims,

We claim as our invention:

1. A control apparatus for an aircraft having a control surface adapted to change the angular position of the aircraft about an axis, and including: motor means operable to position said control surface; means responsive to the angular position of said aircraft about said axis; a first variable impedance controlled by said responsive means; a manually controlled impedance; a follow up type control system connected to said motor means and including said first variable impedance and said manually operated impedance, said control system being effective to control the operation of said motor means in accordance with signals provided by said first variable impedance and by said manually operated impedance; and switching means adapted to render said manual control means ineffective and said first variable impedance effective to control the operation of said motor in such a manner as to tend to maintain said aircraft in a predetermined angular position about said axis,

or to render said manually operated impedance effectivey to produce a signal in accordance with the operation of said impedance, and said first variable impedance effective to produce a signal tending to maintain said aircraft in a given angular position about said axis, whereby said manually operated impedance may be operated to modify the effect of said first variable impedance and thereby change the angular position which said first variable impedance will tend to maintain.

2. A control apparatus for an aircraft having a control surface adapted to change the angular position of the aircraft about an axis and including: motor means operable to position said control surface; means responsive to the angular position of said aircraft about said axis; a rst variable resistor controlled by said responsive means; a manually controlled variable resistor; a rebalancing variable resistor controlled by said motor means; a control circuit including said first resistor, said manually controlled resistor and said rebalancing resistor and adapted to cause signals to be developed by said resistors which may be used to control the operation of said motor means, said rst resistor providing a signal tending to maintain said aircraft in a predetermined angular position about said axis, and said manually operated resistor providing a signal tending to operate said motor means without regard for theangular position of said aircraft about said axis; and switching means operable to render both said first resistor and said manually controlled resistor effective to control the operation of said motor means whereby said first resistor provides a signal to maintain said aircraft in a given angular position about said axis and said manually operated resistor is effective to modify the action of said first resistor to change the angular position which said first resistor tends to maintain, or to render said first resistor ineffective and said manually operated resistor effective to control the operation of said motor means whereby said motor means is operated in accordance with the operation of said manually operated resistor.

3. A control apparatus for an aircraft having a control surface adapted to change the position of the aircraft about an axis and including: motor means operable to position said control surface; means responsive to the angular position of said aircraft about said axis; a first variable resistor controlled by said responsive means; a. manually controlled variable resistor; a rebalancing variable resistor controlled by said motor means; a control circuit of the rebalancing type operable to control the operation of said motor means and including said first variable resistor, said manually controlled variable resistor, and said rebalancing resistor, said first resistor providing a signal which tends to operate said motor means so as to maintain said aircraft in a predetermined angular position about its axis, and said manually controlled resistor providing a signal which normally tends to operate said motor means; and switching means operable to render said manually controlled resistor ineffective and said first resistor effective to control the operation of said motor means, or to render said manually operated means effective to control the operation of said motor means while effectively disconnecting said first variable resistor from said control circuit to render it ineffective to control said motor means.

4. A control apparatus for an aircraft having a control surface adapted to change the angular position of the aircraft about an axis and including: motor means operable to position said control surface; means responsive to the angular 2,5 cocco d5 position of vsaid aircraft .about said axis; a first variable resistor Voperated by ,said responsive means;- a manually variable resistor; a rebalancing `resistor operatedby said motor means; a control circuit connected to said motor means and including said first variable resistor, said manually variable resistor, and said rebalancing resistor and adapted to control the operation of said motor means in accordance with signals provided by said resistors, .said circuit having two alternate connections, one of said connections including said first resistor in the effective portion of said control circuit and vsaid second alternate connectionnot including said first resistor in the effective portion `of said control circuit; and manual switching .means selectively operable ,to vrender said manually variable resistor ineffective to Vprovide .a signal controlling the operation vof said motor .means and to complete said circuit through said first alternate connection whereby said first resistor is effective to control Ythe operation of said motor means and thereby tend to'maintainsaid aircraft vin a predetermined angular position about said axis, or to render said manually variable resistor effective to provide a signal controlling the .operation of said motor means .and to complete said circuit through said rs't .alternate connection whereby said rst resistor .is effective to control the .operation of said motor means and thereby tend to maintain said aircraft in a predetermined angular position about said axis, which position may be 'varied by the .operation of said manually variable resistor, or to render said manually variable resistor effective to provide as'ignal controlincluding: .motor means .operable to position said L.

control surface.; means responsive to the angular position of said aircraft-about .said axis; a first variable .resistor controlled by said responsive means; a manually Icontrolled variable resistor; a rebalancing variable resistorcontrolled by the operation of 4said motor means; a control circuit of the rebalancing `type connected to said motor means to control the opera-tion thereof and including `said first variable resistor, said manually controlled variable resistor and said rebalancing resistor, said first resistor being connected insa-id .circuit so as to provide a signal controlling 'the operation of said motor means in such a manner as to tend to maintain said aircraft in a predetermined angular position about said `axis and said manually operated variable resistor being connected to provide a signal causing the operation .of said motor means in accordance with the operation of said variable resistor, said `circuit having an alternate connection whereby the signals from said rst resistor may be omitted from the signals causing the operation of said motor means; and switching means selectively operable to render said manually controlled resistor `effective to control the operation of said motor means and connect said motor means to said alternate terminals whereby said first resistor is rendered ineffective to control the operation `of said motor means,` or to render said manually controlled resistor ineffect-ive and to establish the normal connection to said motor means whereby said rst resistor is eifective tocontrol the operation of said motor means, or to render said manually .controlled resistor effective and to `establish normal connection to said motor means whereby said first resistor is effective to cause said motor means to operate to tend to maintain said vaircraft, in a predetermined angular position about said axis and said manual control means is eifective to vary said predetermined angular position, or to render said manually operated control means effective to a reduced degree and to establish said normal connection to said motor means whereby said first control means is eiiective .to operate said motor means so as to tend to maintain said aircraft in a predetermined angular position about said axis and said manually? operated means is effective to a reduced degree to vary said predetermined angular position.

6. A control apparatus for an aircraft having first and second control surfaces adapted to change the angular position of the aircraft about a first and second axis respectively, and including: control means associated with each of said control surfaces and including a controller responsive to the angular position of said aircraft about the axis about which said associated control surface is effective, and a manually operated control means, said responsively operated controller being capable of operating said control surface so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manuallyoperated controller being capable of operating said control surface in accordance with the operation of said controller; and switch means operable to render, in each control means, the responsively operated controller effective and the manually operated controller inective vto cause the operation of the associated control surface, or to render the Vresponsively operated controller in each said control means edective, the manually operated controller in said control means associated with said first control surface ineffective, and the manually operated controller in said control means associated with said second control surface effective, whereby said second manually operated controller may be operated to vary the predetermined angular position about said second axis which said -c'orresponding responsively operated controller tends to maintain.

7. A control apparatus for an aircraft having first and second control surfaces adapted to change the angular position of the aircraft about a first and second axis respectively, and including: rst and second motor vmeans operable to change the position of said first and second control surfaces respectively; motor control means for each of said motor means, each said motor control means including a controller responsive to the angular position of said aircraft about the axis about which said associated control surface is effective, and a manually operated conn trol means, said responsively operated controller being capable of operating said motor means so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manually .operated controller being capable of operating said motor means in accordance with the operation of said controller; and .switch Lmeans selectively operable to render, in each motor control means, the responsively operated controller effective and .the manually operated controller ineffective to .cause the operation of the associated motor means, or to render the responsively operated controller in each said motor control means effective, the manuallly operated controller in said motor control means associated with said first motor means ineffective, and the manually operated controller in said motor control means associated with said second motor means effective, whereby said second manually operated controller` may be operated to vary the predetermined angular position about said second axis which said corresponding responsively operated controller tends to maintain.

8. A control apparatus for an aircraft having first and second controlI surfaces adapted to change the angular position of the aircraft about a rst and second axis respectively, and including: control means associated with each of said control surfaces and including a controller responsive to the angular position of said aircraft about the axis about which said associated control surface is effective, and a manually operated control means, said responsively operated controller being capable of operating said control surface so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manually operated controller being capable of operating said control surface in accordance with the operation of said controller; and switching means operable to render the responsively operated controller and the manually operated controller in each said control means effective to cause the operation of the associated control surface, whereby said manually operated controllers may be operated to vary the predetermincd angular positions which said responsively operated controllers tend to maintain, or to render the responsively operated controller in each said control means effective, the manually operated controller in said control means associated with said first control surface ineffective, and the manuallly operated controller in said control means associated with said second control surface effective, whereby said second manually operated controller may be operated to vary the predetermined angular position about said second axis which said corresponding responsively operated controller tends to maintain.

9. A control apparatus for an aircraft having first and second control surfaces adapted to change Athe angular position of the aircraft about a first and second axis respectively, and including: first and second motor means operable to change the position of said first and second control surfaces respectively; motor control means for each of said motor means, each said motor control means including a controller responsive to the angular position of said aircraft about the axis about which said associated control surface is effective, and a manually operated control means, said responsively operated controller being capable of operating said motor means so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manually operated controller bemg capable of operatingsaid motor means in accordance with the operation of said controller; and switching means selectively operable to render the responsively operated controller and the manually operated controller in each said motor control means effective to cause the operation of the associated motor means, whereby said manually operated controllers may be operated to vary the predetermined angular positions which said responsively operated controllers tend to maintain. or to render the responsively operated controller in each said motor control means effective, the manually operated controller in said motor control means associated with said first motor means ineffective, and the manually operated controller in said motor control means associated with said second motor means effective, whereby said second manually operated controller may be operated to vary the predetermined angular position about said second axis which said corresponding responsively operated controller tends to maintain.

l0. A control apparatus for an aircraft having first and second control surfaces adapted to change the angular position of the aircraft about a first and second axis respectively, and including: control means associated with each of said control surfaces and including a controller responsive to the angulara position of said aircraft about the axis about which said associated control surface is effective, and a manually operated control means, said responsively operated controller being capable of operating said control surface so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manually operated controller being capable of operating said control surface in accordance with the operation of said controller; and switching means operable to render, in each control means, the manually operated controller effective and the responsively operated controller ineffective to cause the operation of the associated control surface, or to render the responsively operated controller in each said control means effective, the manually operated controller in said control means associated with said first control surface ineffective, and the manually operated controller in said control means associated with said second control surface effective, whereby said second manually operated controller may be operated to vary the predetermined angular position about said second axis which said corresponding responsively operated conf troller tends to maintain.

11. A control apparatus for an aircraft having first and second control surfaces adapted to change the angular position of the aircraft about a first and second axis respectively, and including: first and second motor means operable to change the position of said first and second control surfaces respectively; motor control means for each of said motor means, each said motor control means including a controller responsive to the angular' position of said aircraft about the axis about which said associated control surface is effective, and a manually operated control means, said responsively operated controller being capable of operating said motor means so as to tend to maintain said aircraft in a predetermined angular position about said axis, and said manually operated controller being capable of operating said motor means in accordance with the operation of said controller; and switching means selectively operable to render, in each motor control means, the manually operated controller effective and the responsively operated controller ineffective to cause the operation of the associated motor means, or to render vthe responsively operated controller in each said motor control means effective, the manually operated controller in said motor control means associated with said first motor means ineffective, and the manually operated controller in said motor control means associated with said second motor means effective, whereby said second manually operated controller may be operated to vary the predetermined angular position about 

